Because of the relationship between torque and RPM in many rotational power sources such as motors, engines, turbines, etc., different usable gear ratios are provided by drive trains connecting such power sources to a useful load to propel a machine, drive an implement or conduct other work. The drive train relies on the relationship between output torque and angular speed (in revolutions per minute “RPM”) to operate at a given gear ratio. Typically, the speed of the power source and/or the load determines the appropriate gear ratio for driving the load. In many systems, a drive train or transmission controller executes this selection automatically without operator intervention.
The automatic selection of gears ratios, while often convenient from an operator standpoint, can lead to gear train oscillations. Such oscillations occur when conditions immediately after a shift indicate that the immediately prior gear ratio should be selected instead. For example, consider a system wherein engine speed is used to determine an appropriate shift point between two gear ratios. When the shift point is reached during acceleration, i.e., increasing engine speed, the shift to a higher gear ratio will cause the engine speed to decrease to a speed lower than the shift point. This in turn, will cause the controller to execute a shift back to the lower gear. However, now the engine will unload and the engine speed will increase past the shift point, causing the controller to execute a shift back to the higher gear.
Such oscillation can be inefficient and annoying, and may decrease the useful life of the drive train and the engine between rebuilds. Accordingly, certain remedial measures have developed in the art to address the issue of gear train oscillation in speed-controlled transmissions. Typically, the transmission controller implements a hysteresis algorithm whereby the shift point between gears is bifurcated into an up-shift point and a down-shift point, with the up-shift point being at a higher engine speed than the down-shift point. In this way, if the engine bogs down slightly after an up-shift, there will not be an immediate down-shift unless the engine loads to such an extent that the lower down-shift point is passed. Similarly, when the engine unloads after a down-shift, there will not be an immediate up-shift unless the engine unloads to such an extent that the higher up-shift point is passed.
The use of hysteresis shift algorithms, however, is not effective for controlling oscillations in parallel path variable transmissions. In these transmissions, a variable direction hydrostatic element drives the gear train such as in certain split torque transmissions. One example of a hydrostatic transmission consisting of a variable speed hydraulic pump and a hydraulic motor is disclosed in U.S. Pat. Nos. 6,385,970 and 6,424,902 to Kuras et al.
In such a transmission, shift points result in a reversal in the direction of the variator acceleration. Thus, the shift points are necessarily defined such that the down-shift and up-shift points between any two ratios are singular, i.e. they both lie at essentially the same RPM (or transmission ratio). This is in contrast to other transmission types as discussed above wherein a shift may properly occur over a broader range of power source speeds. Thus, the anti-hunt hysteresis algorithm employed in many transmission types is generally impractical in parallel path variable transmissions.
The foregoing background discussion is intended solely to aid the reader. It is not intended to be limiting, and thus should not be taken to indicate that any particular element of a prior system is unsuitable for use, nor is it intended to indicate any element, including solving the motivating problem, to be essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims.